Computerized intertemporal choice task in chimpanzees (Pan troglodytes) with/without postreward delay

Abstract

In intertemporal choice (ITC) tasks, animals are presented with alternative choices between a smaller reward that becomes available sooner and a larger reward that becomes available later. To equate the duration of a trial across the 2 options, postreward delays (PRDs) are inserted after the delivery of the reward. Animals need to incorporate this to increase the long-term reward rate. However, recent studies suggest that they have difficulty understanding the contingency associated with PRDs. Previous research indicates that chimpanzees exhibit particularly great self-control in ITC tasks, but it remains unclear whether chimpanzees do so when considering PRDs. Therefore, we used touchscreen experiments to explore chimpanzee intertemporal preferences when trial duration was equated by a PRD as well as when the PRD was eliminated. The computerized setting was used to try to control delay length flexibly and precisely while reducing the impact of the interaction with human experimenters. Moreover, choice options were presented on touchscreens using symbolic cues. This may reduce the impact of seeing food rewards on making a choice (i.e., the animals’ robust tendency to reach for the larger amount of food). In an ITC task in which the trial duration was equated, 4 chimpanzees preferred larger rewards but chose smaller rewards more often when the ratio of the reward amount was smaller. In an ITC task with no PRDs, 2 of 4 chimpanzees did switch their preference to smaller rewards and enhanced the reward rate although this result should be interpreted in light of some methodological limitations.